A main shaft of an electric generator driven by a gas turbine engine generally comprises a plurality of individually formed component parts such as a compressor wheel, a turbine wheel, a support structure for permanent magnets, a plurality of journals, thrust plates, etc., and these component parts are joined together on a common axis. Conventionally, the joint of the component parts is typically achieved by passing a single rod member through the component parts to be joined together, and then applying an axial load upon abutting surfaces of the component parts on the rod member by tightening a bolt nut at an end of the rod member (see U.S. Pat. No. 5,964,663, FIG. 7, for example).
In the conventional rotor shaft disclosed in U.S. Pat. No. 5,964,663, however, some of the component parts constituting the rotor shaft are equipped with a fitting portion at a position relatively close to an inner periphery and some of the fitting portions tend to have a large axial dimension. In addition, fitting portions often have different radial dimensions (or thicknesses). As a result, uneven axial loads may act upon abutting surfaces of the adjacent component parts, and varying amounts of interference may be created between the fitting portions. Further, the amount of interference can decrease during operation due to thermal expansion and/or centrifugal expansion of the component parts, leading to a reduced fitting strength. Consequently, the produced rotor shafts may have different natural frequencies one from another and/or an excessive oscillation can take place at a resonance point, making it difficult to suppress oscillation during operation. Particularly, in a rotor shaft utilizing an air bearing as a radial bearing in order to achieve a higher rotational speed, a restraining force acting upon the rotor shaft tends to be relatively small, and thus, if the axial load acting upon the abutting surfaces of the mating component parts is insufficient, an oscillation can become excessively large when a primary bending resonance frequency is reached as the rotation of the rotor shaft is accelerated. This makes it difficult for the rotor to pass through the resonance frequency safely.
In order to solve such problems, it is necessary to ensure a sufficient interference between fitting portions of the mating members during operation and at the same time apply a sufficiently large axial load upon the abutting surfaces between the mating component parts, although it was quite difficult to meet the requirements in the conventional structure.